24 foot wingspan fff beast -thread 2

Basically, here's the deal:
I'm building a 24 foot wingspan plane made out of fan-fold foam. I have the fuselage and wings almost done, but they are stuck at school, and I'm on break until the middle of January.

I'm building my own motor for it so this should be cheap. A bunch of people have donated a couple dollars each for a lipo fund.

Matt- (asking about difference between 9.6:1 and 6:1 aspect ratio)
A glider guy would probably be able to answer this better, but the way I understand it is this:
The higher the aspect ratio, the more efficient the wing is. This is because, if you look at the pressures at all points along the airfoil, the biggest pressure difference between the top and bottom (and therefore lift) is just slightly behind the leading edge of the wing. This means that increasing the wingspan and decreasing the chord will give you more leading edge, and therefore more lift, for the same amount of wing area.

Some gliders have crazy aspect ratios of 30:1 or more. This isn't going to help you turn faster- that's why aerobatic planes have a small aspect ratio. It will help create nice, smooth, efficient flight though.

Anyone- feel free to speak up if I'm wrong with any of that...

In any case, I'm sure your 6 footer with a 12 inch chord will fly fine. I had a Herr Starlite quite a while ago and it flew great.

Thanks Hans. That sheds a lot of light on it for me. Makes perfect sense now.

I was talking to a glider pilot a while back about the turning abilities of gliders (full scale) and he said that he could beat ANY aerobatic plane in the ribbon cutting contest because his glider could turn on its wing tips just about. His reasoning was because the glider will turn flatter and thus lost less altitude than the aerobatic plane (because they have to rotate along the X axis in order to turn sharper and thus loose lift and then altitude). So that coupled with your comment tells me that a higher aspect ratio plane will act as a better AP platform than one with a lower ratio, because you can make turns that are more flat and thus loose less altitude and keep the camera closer to the desired aiming direction.

Dang it. Wish I would have figured that out BEFORE I cut the wing down to 6 feet. I believe the 8 footer would have yielded better results for what I am after. Oh well, maybe on the next batch of BlueCor I will build to your specs.

...This is because, if you look at the pressures at all points along the airfoil, the biggest pressure difference between the top and bottom (and therefore lift) is just slightly behind the leading edge of the wing. This means that increasing the wingspan and decreasing the chord will give you more leading edge, and therefore more lift, for the same amount of wing area.

This isn't true.
Increasing the span does not give you more LE area just more length. Also no part of the airfoil operates independently of the rest allowing it to be treated in isolation (at least not in subsonic flight - in supersonic flight what happens downstream has no affect because pressure waves can't propagate upstream.)
At the wingtips there is a 'leakage' of pressure from the lower to upper surfaces. This is what causes the vortices off the tips. You've probably seen these vortices illustrated by smoke off the tires of a landing airliner, cloud deformation as a plane passes through it or condensation swirl off the tip of a combat aircraft. Making the tips a smaller part of the wing reduces spanwise flow and tip losses thus improving efficiency.

Lower aspect ratio is advantageous for manoeuverability because the moment of inertia of the plane about it's axis is reduced for the same wing area. Also it is easier to built a strong enough low AR wing for high G manoeuvers than it is to build a high AR wing for the same conditions.

You misunderstand me, I think. My comments refer to wings of the same area and varying aspect ratio. If you increase the span you decrease the chord so that the area remains constant. Assuming the airfoil is the same and changes in reynolds no.s are small the pressure distribution will be similar but at a smaller scale over a longer span. Nett effect is zero until you take into account tip effects. The length of the LE is not in itself a factor.
If I were not talking about constant area then the whole thing would be nonesense - then it's just a discussion about making the plane bigger or smaller.

Quote:

Originally Posted by Charleyh

...Basically, the longer the wing the more lift but also more drag. The shorter the wing, less lift but less drag.

Fighters have short fat wings for speed but have a high stall speed, while cargo planes have long thinner wings and slower stall speeds...

This is incorrect (again assuming that the comparison is for wings of similar area).
Higher aspect ratio wings will have better L/D characteristics all else being equal. The shorter wings on fighters are not there to reduce drag - they do the opposite. There are several reasons why they usually have low aspect ratio wings. To increase wing volume for fuel storage, to make use of high sweep to aid supersonic flight regimes and to decrease moment of inertia and increase strength for high load manoeuvers.
Stall speed is not really a function of the wing planform. A given airfoil at given reynolds no, temperature and pressure will stall at a specific angle of attack even if the wing is infinitely long. The sweep and spanwise flow aspects of the planform will effect this but for operational aircraft the stall speed is determined primarily by the airfoil, high lift devices and wing loading.

The differences you've mentioned between fighters and cargo planes are due to airfoil shape (fighters generally have sharp LE for supersonic flight), wing loading, manoeuverability, structural issues, volume requirements and cost among others. There is no simple relationship between speed and aspect ratio.

Websites:
I don't know many websites. I think there are quite a few about but I don't know many.
Have a look at these two:
Martin Hepperle's site: http://www.mh-aerotools.de/airfoils/index.htm
University of Illinois (Michael Selig): http://www.aae.uiuc.edu/m-selig/
They are interesting sites but with a limited amount of educational stuff - it's mostly data, a lot of it for model aircraft

Books:
John D. Anderson has written several books which seem to be the recommended reading for a lot of post grad aerodynamics. If you have an engineering, maths or physics background you should be able to get to grips with them.'Introduction to Flight'
'Fundamentals of Aerodynamics'
'Aircraft Performance and Design'
'Computational Fluid Dynamics'
'Modern Compressible Fluid Flow'

SoaringDragon, if you do a search on Mark Drela you will probably come up with some information about low speed aerodynamics. At least in the glider/powered glider world, Mark has developed some incredibly optimized airfoils for low reynolds numbers.

Websites:
I don't know many websites. I think there are quite a few about but I don't know many.
Have a look at these two:
Martin Hepperle's site: http://www.mh-aerotools.de/airfoils/index.htm
University of Illinois (Michael Selig): http://www.aae.uiuc.edu/m-selig/
They are interesting sites but with a limited amount of educational stuff - it's mostly data, a lot of it for model aircraft

Books:
John D. Anderson has written several books which seem to be the recommended reading for a lot of post grad aerodynamics. If you have an engineering, maths or physics background you should be able to get to grips with them.'Introduction to Flight'
'Fundamentals of Aerodynamics'
'Aircraft Performance and Design'
'Computational Fluid Dynamics'
'Modern Compressible Fluid Flow'